Plasma processing method

ABSTRACT

To improve the processing rate and uniformity in a plasma processing for a substrate having a relatively large area, a plasma processing apparatus includes a reaction vessel which has a portion made of a dielectric member, which accommodates a film formation substrate, and which can be evacuated, an evacuating means and a gas supply means for supplying a predetermined gas into the reaction vessel, a cathode electrode arranged in a position outside the reaction vessel where the cathode electrode opposes the film formation substrate accommodated in the reaction vessel via the dielectric member, and a high frequency power supply means (a matching circuit and a high frequency power supply) for supplying high frequency power of 30 MHz to 300 MHz to the cathode electrode. The high frequency power of 30 MHz to 300 MHz is supplied to the cathode electrode to generate a plasma between the dielectric member and the film formation substrate.

This application is a division of application Ser. No. 08/791,460, filedJan. 27, 1997 now U.S. Pat. No. 5,970,907.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a plasma processing apparatus and aplasma processing method which can be suitably used as, e.g., a plasmaCVD apparatus useful to semiconductor devices such as anelectrophotographic photosensitive device, an image input line sensor,an image pickup device, and a photovoltaic device, a sputteringapparatus for forming, e.g., insulating films and metal interconnectinglines as semiconductor devices and optical elements, and an etchingapparatus for semiconductor devices and the like.

2. Related Background Art

In the fabrication of semiconductors and the like devices, variousplasma processing methods and apparatuses are used in accordance withthe intended uses. For example, apparatuses and methods using thecharacteristics of plasma are used in, e.g., the formation of oxidefilms, nitride films, and amorphous silicon semiconductor films using aplasma CVD process, the formation of metal interconnecting layers usinga sputtering process, and the fine processing techniques using etching.Also, as demands on the film quality and the performance are increasingrecently, various improvements are being examined. In particular, aplasma process using high frequency power is extensively used because ofthe advantages that 1) discharge is stable, and 2) process can beapplied to insulating materials such as an oxide film and a nitridefilm.

As one example of a plasma CVD apparatus generally used in the formationof deposited films, FIG. 1 shows a plasma processing apparatus as a filmformation apparatus for an amorphous silicon film (to be referred to asan a-Si film hereinafter) for a cylindrical electrophotographicphotosensitive body.

In FIG. 1, the apparatus comprises a reaction vessel 201 which can beevacuated. This reaction vessel 201 is connected to an evacuating means209 for evacuating the vessel and a gas supply means 210 for supplying agas into the vessel.

In the reaction vessel 201, a cylindrical cathode electrode 202electrically insulated from the reaction vessel 201 by an insulatingmaterial 211 is arranged. Additionally, a cylindrical film formationsubstrate 203 is arranged as a counter electrode inside the cathodeelectrode 202. The film formation substrate 203 is held by a substrateholder 204 having a rotating mechanism driven by a motor 212. A heater205 is positioned in the internal space of the film formation substrate203. The film formation substrate 203 can be heated to a predeterminedtemperature from the inside by the heater 205 arranged in the internalspace. The cathode electrode 202 is connected to a high frequency powersupply 207 for discharge via a matching circuit 208.

Note that the oscillation frequency of a discharge high frequency powersupply used in a plasma process such as plasma CVD is commonly 13.56MHz. The oscillation frequency of the high frequency power supply 207 isalso 13.56 MHz.

An a-Si film formation method using this plasma processing apparatuswill be described below.

First, the reaction vessel 201 is evacuated to a high vacuum by theevacuating means 209. Thereafter, the gas supply means 210 supplies asource gas such as silane gas, disilane gas, methane gas, or ethane gasand a doping gas such as diborane gas to maintain the pressure atseveral tens of millitorr to a few torr. Subsequently, the highfrequency power supply 207 supplies high frequency power of 13.56 MHz tothe cathode electrode 202 to generate a plasma between the cathodeelectrode 202 and the film formation substrate 203, thereby decomposingthe source gas. Consequently, an a-Si film is deposited on the filmformation substrate 203 heated to about 200° C. to 350° C. by the heater205.

In the above apparatus using the 13.56-MHz high frequency power, eventhe maximum deposition rate at which a-Si films meeting the performanceof a recent electrophotographic photosensitive body can be formed is atmost about 6 (μm/hour). If the deposition rate is further increased, itis sometimes impossible to obtain satisfactory characteristics as aphotosensitive body. Generally, when an a-Si film is used as anelectrophotographic photosensitive body, a film thickness of at least 20to 30 μm is necessary to obtain charging power. Accordingly, a longprocessing time is required to manufacture an electrophotographicphotosensitive body meeting the required performance.

As a method of increasing the deposition rate, a plasma CVD process(Plasma Chemistry and Plasma Processing, Vol. 7, No. 3, (1987) pp.267-273) is reported. This technique suggests that the deposition ratecan be increased without lowering the performance of the deposited filmby increasing the discharge frequency to be higher than 13.56 MHz byusing a high frequency power supply with a frequency higher than 13.56MHz. This method of increasing the discharge frequency is performed inthe field of sputtering and the like and has been extensively studied.

As described above, the conventional plasma processing apparatus has thedrawback that a long processing time is required to manufacture anelectrophotographic photosensitive body and the like. Therefore, it isnecessary to decrease the deposition time without lowering theperformance of the deposited film.

Unfortunately, it is found that if a frequency higher than 13.36 MHz,e.g., a high frequency of 100 MHz, is directly applied to theconventional apparatus, the deposition rate of the deposited filmvaries, or etching cannot be evenly performed. That is, it turns outthat it is difficult to perform uniform plasma processing or filmdeposition on a substrate with a relatively large area.

This degrades various characteristics of the deposited film. Theconventional plasma processing apparatus has many problems to be solvedin order to form a deposited film having uniform characteristics at auniform deposition rate.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve the above conventionalproblems and provide a plasma processing apparatus and a plasmaprocessing method capable of evenly performing plasma processing on asubstrate having a relatively large area at a processing rate whichcannot be achieved by any conventional plasma processing apparatus.

It is another object of the present invention to provide a plasmaprocessing apparatus and a plasma processing method capable of forming adeposited film having uniform or substantially uniform characteristicsin its entirety.

It is still another object of the present invention to provide a plasmaprocessing apparatus and a plasma processing method capable of forming adeposited film at a uniform or substantially uniform deposition rateregardless of the position on a substrate where the film is deposited.

It is still another object of the present invention to provide a plasmaprocessing apparatus and a plasma processing method capable ofperforming etching processing uniformly or substantially uniformly foran object to be etched.

It is still another object of the present invention to provide a plasmaprocessing apparatus or method capable of generating a plasma uniformlyor substantially uniformly.

The present invention provides a plasma processing apparatus comprisinga reaction vessel having a dielectric member in a portion thereof,capable of accommodating a film formation substrate or an object to beprocessed, and capable of being evacuated, gas supply means forsupplying a predetermined gas into the reaction vessel, a cathodeelectrode provided outside the reaction vessel and arranged in aposition where the cathode electrode opposes the film formationsubstrate or the object to be processed accommodated in the reactionvessel via the dielectric member, and high frequency power supply meansfor supplying high frequency power of 30 MHz to 300 MHz to the cathodeelectrode.

The present invention also provides a plasma processing method performedin a plasma processing apparatus comprising a reaction vessel having adielectric member in a portion thereof, capable of accommodating a filmformation substrate or an object to be processed, and capable of beingevacuated, gas supply means for supplying a predetermined gas into thereaction vessel, a cathode electrode provided outside the reactionvessel and arranged in a position where the cathode electrode opposesthe film formation substrate or the object to be processed accommodatedin the reaction vessel via the dielectric member, and high frequencypower supply means for supplying high frequency power of 30 MHz to 300MHz to the cathode electrode, wherein after the predetermined gas issupplied into the reaction vessel, high frequency power of 30 MHz to 300MHz is supplied to the cathode electrode to generate a plasma betweenthe dielectric member and the film formation substrate or the object tobe processed in the reaction vessel, thereby forming a film on the filmformation substrate or performing plasma processing for the object to beprocessed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view for explaining one example of conventionalplasma processing apparatuses;

FIGS. 2 and 5 are schematic views for explaining preferred embodimentsof a plasma processing apparatus of the present invention; and

FIGS. 3, 4, 6, and 7 are schematic perspective views for explainingpreferred embodiments of a cathode electrode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As one means for solving the above conventional problems, it is reportedthat the deposition rate can be increased by using a high frequencypower supply with a frequency higher than 13.56 MHz. To increase thedeposition rate without lowering the performance of the deposited film,the present inventors used a high frequency power supply having adischarge frequency higher than 13.56 MHz in the plasma processingapparatus shown in FIG. 1 and extensively studied this apparatus. Theresults will be described below.

In the conventional plasma processing apparatus shown in FIG. 1, if anelectrophotographic photosensitive body having a diameter of, e.g.,about 100 mm is used as the film formation substrate 203, an electrodehaving a diameter d of 200 to 300 mm is used as the cathode electrode202 accordingly. When a high frequency is supplied from one point on theouter circumferential surface of the cathode electrode 202, the distanceto a point at the opposite side on the outer circumferential surface ofthe cathode electrode 202 is 1.57 d. For example, if d=250 mm, thedistance is about 390 mm. When a high frequency power supply with afrequency of 100 MHz is used, a wavelength k is approximately 3 m in theair. Consequently, in this plasma processing apparatus a high frequencysupplied from one point on the outer circumferential surface of thecathode electrode 202 propagates on the outer circumferential surface ofthe cathode electrode 202 and reaches the opposite side. However, it isfound that a standing wave is generated because the propagation distanceis λ/10 or more and an electric field distribution is formed on theouter circumferential surface of the cathode electrode under theinfluence of this standing wave. It is found that under the influence ofthis electric field, i.e., an RF field, generated on the outercircumferential surface of the cathode electrode, an uneven field isformed on the inner circumferential surface of the cathode electrode 202and this produces uneven discharge in the circumferential direction.

Additionally, in the above plasma processing apparatus, the length ofthe electrophotographic photosensitive body as the film formationsubstrate 203 is usually about 350 mm. Accordingly, the length of thecathode electrode 202 is about 350 to 400 mm. It turns out that, as aconsequence, uneven discharge similar to that in the circumferentialdirection also occurs in the axial direction (longitudinal direction).

From the foregoing, it is found that in the conventional plasmaprocessing apparatus, film deposition can be performed at a higherdeposition rate by increasing the frequency, but uneven discharge occursbecause the discharge frequency is raised, and this poses anotherproblem to be described below which is not brought about at a dischargefrequency of 13.56 MHz.

That is, raising the discharge frequency unevenly distributes the plasmaand makes the deposition rate nonuniform. As a result, in an object tobe processed having a relatively large area, e.g., anelectrophotographic photosensitive body, a film thickness variation (inthe case of, e.g., an electrophotographic photosensitive body, a filmthickness variation of ±20% or more) which is practically undesirableoccurs. This film thickness variation is a serious problem not only inan electrophotographic photosensitive body but also in the formation ofa crystalline or non-single-crystal functional deposited film for usein, e.g., an image input line sensor, an image pickup device, and aphotovoltaic device. Also, even in other plasma processes such as dryetching and sputtering, similar unevenness of processing occurs to posea big problem when the discharge frequency is raised.

The present invention is based on the finding that the above problemscan be solved by a plasma processing apparatus comprising a reactionvessel having a dielectric member in a portion thereof which is capableof accommodating a film formation substrate or an object to beprocessed. It is further capable of being evacuated. Gas supply meansfor supplying a predetermined gas into the reaction vessel, a cathodeelectrode provided outside the reaction vessel and arranged in aposition where the cathode electrode opposes the film formationsubstrate or the object to be processed housed in the reaction vesselvia the dielectric member, and high frequency power supply means forsupplying high frequency power of 30 MHz to 300 MHz to the cathodeelectrode are also included.

In the above plasma processing apparatus, the cathode electrode can alsobe subjected to geometric machining. This geometric machining can beaperture formation or slit formation.

Also, a soft magnetic material can be used in a portion of the cathodeelectrode.

In each of the above plasma processing apparatuses, the cathodeelectrode can also be a cylindrical electrode. If this is the case, thefilm formation substrate can be a cylindrical substrate, and the filmformation substrate and the cathode electrode can be coaxially arranged.

The cathode electrode and the film formation substrate or the object tobe processed can also be flat plates opposing each other.

The present invention is also based on the finding that the aboveconventional problems can be solved by a plasma processing methodperformed in any of the above plasma processing apparatuses, whereinafter a predetermined gas is supplied into the reaction vessel, highfrequency power of 30 MHz to 300 MHz is supplied to the cathodeelectrode to generate a plasma between the dielectric member and thefilm formation substrate or the object to be processed in the reactionvessel, thereby forming a film on the film formation substrate orperforming plasma processing for the object to be processed.

The deposition rate can be increased by raising the discharge frequencyto be higher than 13.56 MHz. However, if the discharge frequency ishigher than 13.56 MHz, a standing wave which causes uneven discharge isgenerated on the outer circumferential surface of the cathode electrode.Under the influence of this standing wave, nonuniformity of intensityoccurs in the plasma. Therefore, the apparatus must be so constructedthat the RF standing wave which causes the uneven RF voltage on thecathode electrode is not reflected on the uneven intensity of plasma.Also, the distribution of the plasma is sensitive to the shape of thecathode electrode. Accordingly, to prevent unevenness of the highfrequency power supplied on the surface of the cathode electrode, it isnecessary to adjust the distribution of the high frequency propagatingon the rear surface or the front surface of the cathode electrode.

In the plasma processing apparatus and the plasma processing methodaccording to the present invention, the cathode electrode and the filmformation substrate, or the object to be processed, are opposed to eachother via the dielectric member constituting part of the reactionvessel. Therefore, the high frequency power from the cathode electrodeis supplied via the dielectric member, so a plasma is generated betweenthe dielectric member and the film formation substrate or the object tobe processed. In the plasma processing apparatus in which the highfrequency power is supplied from the cathode electrode via thedielectric member as described above, the unevenness of dischargeoccurring on the inner circumferential surface of the cathode electrodeunder the influence of the standing wave can be reduced by thedielectric member. This prevents a film thickness variation or unevenetching.

Also, since the cathode electrode is arranged outside the reactionvessel, the shape and material of the cathode electrode can be freelychanged without opening the reaction vessel. Accordingly, the RFdistribution on the cathode electrode can be adjusted by changing thecomplex impedance in an arbitrary position of the cathode electrode bychanging the shape and material of the cathode electrode. The result isa substantially uniform potential distribution on the cathode electrode.Furthermore, an optimum shape and an optimum material of the cathodeelectrode change in accordance with the shape of the film formationsubstrate, the plasma processing conditions, and the dischargefrequency. Since the cathode electrode is arranged outside the reactionvessel as described above, only the cathode electrode needs to bereplaced; that is, it is unnecessary to open the reaction vessel intothe air. Consequently, it is readily possible to control changes invarious processing conditions. For a similar reason, an optimum cathodeelectrode can be easily determined by trial and error. Even if a slightpotential distribution is formed on the cathode electrode, thedielectric member is arranged between the cathode electrode and plasmaand the buffering effect of this dielectric material makes the plasmadistribution more uniform than the potential distribution on the cathodeelectrode.

It is experimentally confirmed that a plasma density variation (which isdefined as a value obtained by dividing the difference between themaximum and minimum values of the plasma density by its average value)is ±10% near 30 MHz and this significantly increases the RF voltageunevenness on the cathode electrode caused by the discharge frequency. Afrequency exceeding 300 MHz is impractical because this makes the RFmatching circuit difficult to design and increases the transmissionloss. Also, the energy of ions incident on the film formation substrateis about 30 eV at 13.56 MHz, about 15 eV at 30 MHz, and about 10 eV at100 MHz. In a process using the energy of incident ions onto the filmformation substrate, decreasing the width of this energy is importantsince the controllability can be improved. Therefore, the use of afrequency of 30 MHz or higher is desirable. In the plasma processingapparatus and the plasma processing method of the present invention, ahigh frequency of 30 MHz to 300 MHz is used by taking these factors intoconsideration. Accordingly, it is possible to provide a practical plasmaprocessing apparatus in which the controllability of the RF voltagedistribution on the cathode electrode is excellent, the degree offreedom of design of an RF matching circuit is high, and thetransmission loss is small.

Embodiments of the present invention will be described below withreference to the accompanying drawings.

A plasma processing apparatus of the present invention is characterizedin that a high frequency power supply which supplies power with afrequency higher than 13.56 MHz is used, the shape of a cathodeelectrode can be freely changed so as to suppress generation of astanding wave, and a buffering function is provided between the standingwave on the surface of the cathode electrode and plasma so that the RFstanding wave which causes an uneven RF voltage on the cathode electrodeis not reflected in unevenness of the intensity of the plasma.

FIG. 2 is a schematic view showing the outline of a cylindrical coaxialplasma CVD apparatus as a plasma processing apparatus according to thefirst embodiment of the present invention.

Referring to FIG. 2, a reaction vessel 1 is a vessel capable of beingevacuated and a portion of the vessel is a dielectric tube made of adielectric member 11. This reaction vessel 1 is connected to anevacuating means 9, as a gas introducing means, for evacuating thevessel and a gas supply means 10 for supplying a predetermined gas intothe vessel.

A cylindrical cathode electrode 2 electrically insulated from thereaction vessel 1 is arranged outside the dielectric tube of thereaction vessel 1. A grounding shield 6 is formed outside the cathodeelectrode 2. The reaction vessel 1 accommodates a cylindrical filmformation substrate 3 as a counter electrode. The cylindrical cathodeelectrode 2 and the film formation substrate 3 are coaxially arranged.The cylindrical cathode electrode 2 is connected to a discharge highfrequency power supply 7 via a matching circuit 8. The film formationsubstrate 3 is held by a substrate holder 4 having a rotating mechanismdriven by a motor 12. The film formation substrate 3 incorporates aheater 5 and can be heated to a predetermined temperature from theinside by the heater 5.

The dielectric member 11 can be made from any material as long as theloss of a high frequency is small. For example, it is possible to usealumina ceramics, quartz glass, pyrex, and Teflon. Since the dielectricmember 11 is used as a portion of the reaction vessel 1 capable of beingevacuated and the reaction vessel 1 is evacuated, the dielectric member11 must have a thickness which can resist the atmospheric pressure.Although the thickness depends upon the shape and dimensions, it isgenerally at least 5 mm, and desirably 10 mm or more. When thedielectric member 11 with this thickness was arranged between thecathode electrode and plasma while a discharge frequency of 13.56 MHzwas used, a reactance component i/jωC of the complex impedance ascribedto a capacitance C of the dielectric material became 10 to 50 Ω whichwas equivalent to the plasma impedance. Consequently, it was difficultto efficiently supply a high frequency to the plasma. However, since ahigh discharge frequency is used in this embodiment, the compleximpedance attributed to the dielectric member 11 decreases in inverseproportion to the frequency. Accordingly, even when the dielectricmember 11 having the above thickness is arranged between the cathodeelectrode and plasma, a high frequency can be efficiently supplied tothe plasma.

In this embodiment, the cylindrical cathode electrode 2 is arrangedoutside the reaction vessel 1. Therefore, the shape and material of thecylindrical cathode electrode 2 can be largely changed to obtain evendischarge in a large area. Consequently, it is possible to change thecomplex impedance at an arbitrary point on the cylindrical cathodeelectrode 2. Note that an optimum shape and an optimum material of thiscylindrical cathode electrode 2 change in accordance with the shape ofthe film formation substrate, the plasma processing conditions, and thedischarge frequency.

It is experimentally confirmed that a plasma density variation (which isdefined as a value obtained by dividing the difference between themaximum and minimum values of the plasma density by its average value)is ±10% near 30 MHz and this significantly increases the RF voltageunevenness on the cathode electrode caused by the discharge frequency. Afrequency exceeding 300 MHz is impractical because this makes the RFmatching circuit difficult to design and increases the transmissionloss. Also, the energy of ions incident on the film formation substrateis about 30 eV at 13.56 MHz, about 15 eV at 30 MHz, and about 10 eV at100 MHz. In a process using the energy of incident ions onto the filmformation substrate, decreasing the width of this energy is importantsince the controllability can be improved. Therefore, the use of afrequency of 30 MHz or higher is desirable. From the foregoing, theplasma processing apparatus of this embodiment uses a power supplycapable of supplying high frequency power of 30 MHz to 300 MHz as thehigh frequency power supply 7.

To form, e.g., an a-Si film by using this plasma processing apparatus,the reaction vessel 1 is evacuated to a high vacuum by the evacuatingmeans 9. The gas supply means 10 supplies a source gas such as silanegas, disilane gas, methane gas, or ethane gas and a doping gas such asdiborane gas to maintain the pressure at several tens of millitorr to afew torr. Subsequently, the high frequency power supply 7 supplies highfrequency power to the cylindrical cathode electrode 2 to generate aplasma between the dielectric member and the film formation substrate 3,thereby decomposing the source gas supplied into the vessel. At thistime, the film formation substrate 3 is heated to about 200° C. to 350°C. by the heater 5 and an a-Si film is deposited on the film formationsubstrate 3.

When the high frequency power is supplied to the plasma, the matchingcircuit 8 adjusts the impedance of the high frequency power suppliedfrom the high frequency power supply 7 so that the impedance matches theimpedance of the plasma. This high frequency power with the adjustedimpedance is supplied to the rear surface of the cylindrical cathodeelectrode 2. Consequently, the high frequency propagates from the rearsurface to the front surface of the cathode electrode 2 through thesurface layer of the cathode electrode, and the high frequency power issupplied to the plasma. To prevent unevenness of the high frequencypower on the surface of the cylindrical cathode electrode 2, thedistribution of the high frequency propagating on the rear surface orthe front surface of the cylindrical cathode electrode 2 is adjusted.That is, the complex impedance at an arbitrary point on the cylindricalcathode electrode 2 is changed by changing the shape and material of thecylindrical cathode electrode 2, thereby making the plasma processinguniform. More specifically, an optimum shape and an optimum material ofthe cylindrical cathode electrode 2 are obtained by the followingadjustments, and the plasma processing is made uniform accordingly:

1) the length of the cylindrical cathode electrode 2 is adjusted inaccordance with the discharge frequency and the length of the filmformation substrate.

2) apertures or slits are formed in the cylindrical cathode electrode 2so that the high frequency power propagates from the rear surface of thecathode electrode through these apertures or slits. The size of theseapertures or slits is adjusted.

3) a soft magnetic material having a high permeability, e.g., permalloy,is used in a portion of the cylindrical cathode electrode 2, and aninductance L in that portion is increased to suppress the transmissionof the high frequency power. In this manner the transmission of the highfrequency power in the cylindrical cathode electrode 2 is adjusted.

Note that the step of obtaining an optimum shape and an optimum materialof the cylindrical cathode electrode 2 need not be performed if unevendischarge occurring on the inner circumferential surface of thecylindrical cathode electrode 2 is removed by the buffering action ofthe dielectric member 11.

<Example 1> to <Example 5> will be described below as practical examplesof the plasma processing apparatus of the above first embodiment, andthe results of comparison with a conventional apparatus will beexplained.

EXAMPLE 1

In this example, the discharge frequency was set to 100 MHz, and an a-Sifilm was formed on the film formation substrate 3 under the filmformation conditions shown in Table 1 below.

TABLE 1 Source gas SiH₄ Carrier gas H₂ Gas flow rates SiH₄ 350 sccm H₂350 sccm Pressure 0.03 torr Substrate temperature 250° C. High frequencypower 0.5 W/cm²

An Al simple cylindrical electrode having an inner diameter of 250 mmand a length of 300 mm was used as the cylindrical cathode electrode 2.An alumina ceramics dielectric tube 10 mm thick was used as thedielectric member 11 constituting part of the reaction vessel 1. Notethat when the film formation conditions are changed, the length of thecylindrical cathode electrode 2 needs to be changed accordingly.Therefore, the length of the cylindrical cathode electrode 2 is notrestricted to the one in this example and is desirably properlyselected.

When film formation was performed by using the plasma processingapparatus in which the cylindrical cathode electrode 2 as describedabove was arranged outside the reaction vessel 1, the average filmformation rate was 18 (μm/hr) and the film thickness variation wasapproximately ±15%. In contrast, when film formation was done by using aplasma processing apparatus (conventional apparatus) in which a 300-mmlong Al simple cylindrical cathode electrode was arranged inside thereaction vessel 1, the film thickness variation was approximately ±30%.Consequently, when the cylindrical cathode electrode 2 was arrangedoutside the reaction vessel 1, it was possible to obtain the filmthickness distribution improving effect for the film thicknessvariation.

Additionally, the influence of the thickness distribution was large inthe formed film. The local film quality of the a-Si film was measured inthe same film thickness state, and it was found that the film qualitywas usable as, e.g., an electrophotographic photosensitive device or animage input line sensor.

In the plasma processing apparatus of this example as described above,the cathode electrode is arranged outside the reaction vessel, and ahigh frequency is supplied to the plasma via a dielectric memberconstituting part of the reaction vessel capable of being evacuated.This buffers the influence of the potential distribution of the highfrequency on the cathode electrode. Consequently, no film thicknessvariation is produced even when the discharge frequency is raised.

EXAMPLE 2

In this example, an aperture Al cylinder having a plurality of aperturesformed in its wall as shown in FIG. 3 was used as the cylindricalcathode electrode 2. Following the same procedure as in Example 1, thedischarge frequency was set to 100 MHz and an a-Si film was formed onthe film formation substrate 3 under the film formation conditions inTable 1 described above. Note that when the film formation conditionsare changed, the aperture shape sometimes needs to be changedaccordingly. Note also that similar effects can sometimes be obtainedfrom entirely different shapes. Therefore, the aperture shape is notlimited to the shape shown in FIG. 3 and is desirably properly chosen.

The film thickness variation in the circumferential direction when filmformation was performed by using this aperture cathode electrode wasmeasured and found to be about ±5%. Compared to the value of about ±15%obtained by the simple cylindrical cathode in Example 1, the filmthickness distribution improving effect was enhanced. Also, the averagefilm formation rate was equivalent to that in Example 1.

The influence of the film thickness distribution was large in eachformed film. The local film quality of the a-Si film was measured in thesame film thickness state, and it was found that the film quality wasusable as, e.g., an electrophotographic photosensitive device or animage input line sensor.

In the plasma processing apparatus of this example as described above,the cathode electrode is arranged outside the reaction vessel.Accordingly, it is readily possible to freely control the geometricshape of the cathode electrode without inducing anomalous discharge.Consequently, the path and the standing wave of the cathode electrodefor transmitting a high frequency can be changed, and this permits freecontrol of the RF voltage distribution on the cathode electrode. As aconsequence, it is possible to avoid a film thickness variation when thedischarge frequency is raised.

EXAMPLE 3

In this example, permalloy sheets 13 in a predetermined pattern as shownin FIG. 4 were adhered as a soft magnetic material to the cylindricalcathode electrode 2. Following the same procedure as in Example 1, thedischarge frequency was set to 100 MHz and an a-Si film was formed onthe film formation substrate 3 under the film formation conditions inTable 1 described above. Note that permalloy was used as the softmagnetic material in the cylindrical cathode electrode 2, but any softmagnetic material can be used. Note also that when the film formationconditions are changed, the pattern of the soft magnetic materialsometimes needs to be changed accordingly, and that similar effects cansometimes be obtained from entirely different patterns. Therefore, thepattern of the permalloy sheets is not limited to the pattern shown inFIG. 4 and is desirably appropriately selected.

When film formation was performed by using the cylindrical cathodeelectrode 2 shown in FIG. 4, the average film formation rate wasequivalent to that in Example 1 and the film thickness variation wasapproximately ±5% when the soft magnetic material was adhered toportions of the outer circumferential surface of the cathode. Comparedto the film thickness variation of about ±15% in Example 1, the filmthickness distribution improving effect was enhanced by the use of thesoft magnetic material.

The influence of only the film thickness distribution was large in eachformed film. The local film quality of the a-Si film was measured in thefilm thickness state formed in Example 3, and it was found that the filmquality was usable as, e.g., an electrophotographic photosensitivedevice or an image input line sensor.

In the plasma processing apparatus of this example as described above, asoft magnetic material is used in portions of the cathode electrode andthe complex impedance at a high frequency can be increased accordingly.Consequently, the path of the high frequency can be controlled only in aportion other than the soft magnetic material. Also, as in Example 2described above, the RF voltage distribution on the cathode electrodecan be freely controlled. As a consequence, it is possible to avoid afilm thickness variation when the discharge frequency is raised.

The cylindrical coaxial plasma CVD apparatus in which the cylindricalcathode electrode 2 is arranged outside the reaction vessel 1 has beendescribed above. However, the present invention is readily applicable toa parallel plate plasma processing apparatus provided that thecylindrical cathode electrode 2 can be arranged outside the reactionvessel.

FIG. 5 is a schematic view showing the outline of a parallel plateplasma CVD apparatus as a plasma processing apparatus according to thesecond embodiment of the present invention.

In FIG. 5, a reaction vessel 101 is a vessel capable of being evacuated,and a portion of the vessel is made of a dielectric square member 111.As in the plasma processing apparatus of the first embodiment describedpreviously, the reaction vessel 101 is connected to an evacuating means(not shown), as a gas introducing means, for evacuating the vessel and agas supply means (not shown) for supplying a gas into the vessel.

A plate-like square cathode electrode 102 electrically insulated fromthe reaction vessel 101 is arranged outside the reaction vessel 101. Aplate-like film formation substrate 103 as a counter electrode isarranged inside the reaction vessel 101. These electrodes are opposed toeach other via the dielectric member 111.

The square cathode electrode 102 is provided in the space surrounded bya grounding shield 106 and is connected to a high frequency power supply107 for discharge via a matching circuit 108.

The film formation substrate 103 is placed on a substrate holder 104incorporating a heater 105 and is heated to a predetermined temperatureby the heater 105. A portion of the substrate holder 104 is extended tothe outside of the reaction vessel 101 and grounded.

In this parallel plate plasma CVD apparatus, as in the cylindricalcoaxial plasma CVD apparatus of the first embodiment, high frequencypower is supplied to the square cathode electrode 102 to generate aplasma between the dielectric member 111 and the film formationsubstrate 103, thereby forming an a-Si film on the film formationsubstrate 103. As in the first embodiment, the plasma processing is madeuniform by adjusting the square cathode electrode 102.

<Example 4> to <Example 6> will be described below as practical examplesof the plasma processing apparatus of the second embodiment, and theresults of comparison with a conventional apparatus will be explained.

EXAMPLE 4

In this example, the parallel plate plasma CVD 15 apparatus shown inFIG. 5 was used to form an a-Si film on a film formation substrate bysetting the discharge frequency to 100 MHz under film formationconditions shown in Table 2 below.

TABLE 2 Source gas SiH₄ Carrier gas H₂ Gas flow rates SiH₄ 450 sccm H₂450 sccm Pressure 0.03 torr Substrate temperature 250° C. High frequencypower 0.5 W/cm²

The film formation substrate 103 was a square substrate, and a squareplate of 450 mm side was used as the square cathode electrode 102. Asthe square dielectric member 111, a 20-mm thick alumina ceramics memberwas used.

In this parallel plate plasma CVD apparatus, the distances from theposition where the high frequency was supplied to the square cathodeelectrode 102 to an edge and a corner of the square shape weredifferent, so a film thickness distribution was easily produced. In thisapparatus, the average film formation rate was 15 (μm/hr) and the filmthickness variation was about ±18%. As a comparative example, the squarecathode electrode 102 was arranged inside the reaction vessel 101 andfilm formation was performed under the same conditions as above.Consequently, the film thickness variation was about ±35%. From thisresult the film thickness distribution improving effect was confirmed.

The influence of only the film thickness distribution was large in eachformed film. The local film quality of the a-Si film was measured in thefilm thickness state formed in Example 4, and it was found that the filmquality was usable as, e.g., an electrophotographic photosensitivedevice or an image input line sensor.

EXAMPLE 5

In this example, the square cathode electrode 102 subjected to geometricmachining as shown in FIG. 6 was used. When the film formationconditions are changed, the geometric shape of the cathode electrodesometimes needs to be modified accordingly. Also, similar effects cansometimes be obtained from entirely different shapes. Therefore, thegeometric shape is not limited to the shape shown in FIG. 6 and isdesirably properly selected.

The film thickness variation was measured when film formation wasperformed by using the square cathode electrode 102 having the shapeshown in FIG. 6. Consequently, the average film formation rate was 15(μm/hr) and the film thickness variation was about ±10%. Compared to thefilm thickness variation of about ±18% in Example 4 described above, ahigher film thickness uniformity could be obtained.

The influence of only the distribution was large in each formed film.The local film quality of the a-Si film was measured in the filmthickness state formed in Example 5, and it was found that the filmquality was well practically usable as, e.g., an electrophotographicphotosensitive device or an image input line sensor.

EXAMPLE 6

In this example, permalloy sheets 113 in a pattern shown in FIG. 7 wereadhered to the square cathode electrode 102. Although permalloy was usedas the soft magnetic material, any soft magnetic material can be used.When the film formation conditions are changed, the pattern of the softmagnetic material sometimes needs to be modified accordingly. Also,similar effects can sometimes be obtained from entirely differentpatterns. Therefore, the pattern is not limited to the one shown in FIG.7 and is desirably properly selected.

The film thickness variation was measured when film formation wasperformed with the cathode electrode shown in FIG. 7. As a consequence,the average film formation rate was 15 (μm/hr) and the film thicknessvariation was about ±10%. Compared to the film thickness variation ofabout ±18% in Example 4 described above, a higher film thicknessuniformity could be obtained.

The influence of the film thickness distribution was large in eachformed film. The local film quality of the a-Si film was measured in thefilm thickness state formed in Example 6, and it was found that the filmquality was usable as, e.g., an electrophotographic photosensitivedevice or an image input line sensor.

The present invention has the arrangements as described above and cantherefore achieve the following effects.

In the plasma processing apparatus and the plasma processing method ofthe present invention, a substrate having a relatively large area can beuniformly processed with a plasma at a processing rate which cannot beachieved by any conventional plasma processing apparatus.

Additionally, the cathode electrode is arranged outside the reactionvessel and hence can be replaced without opening the reaction vesselinto the atmosphere. Consequently, it is readily possible to determineby trial and error the optimum shape and the optimum material of thecathode electrode by which no standing wave is produced (or even if astanding wave is produced, its influence is small).

Also, in the present invention the complex impedance at an arbitrarypoint on the cathode electrode can be changed by performing geometricmachining for the cathode electrode. This further improves theuniformity of plasma processing.

Furthermore, in the present invention the complex impedance at anarbitrary point on the cathode electrode can be changed by using a softmagnetic material in a portion of the cathode electrode. This furtherimproves the uniformity of plasma processing.

The present invention is not limited to the above embodiments and isapplicable to various types of plasma processing, such as etching, otherthan film formation. It is of course possible to make modifications andcombinations of the present invention without departing from the spiritand scope of the invention.

What is claimed is:
 1. A plasma processing method performed in a plasmaprocessing apparatus comprising: a vessel having a member for reducinghigh frequency distribution in a portion thereof, capable of housing afilm formation substrate or an object to be processed, and capable ofbeing evacuated; gas supply means for supplying a predetermined gas intothe vessel; a cathode electrode provided outside the vessel and arrangedin a position where the cathode electrode faces the object to beprocessed housed in the vessel, an empty space being provided betweenthe member for reducing high frequency distribution and the cathodeelectrode; and high frequency power supply means for supplying highfrequency power of 30 MHz to 300 MHz to the cathode electrode, themethod comprising the steps of: supplying a predetermined gas into thevessel; supplying high frequency power of 30 MHz to 300 MHz to thecathode electrode to generate a plasma between the member for reducinghigh frequency distribution and the object to be processed in thevessel, the member having a thickness of 5 mm or more, therebyperforming plasma processing for the object to be processed.
 2. Theplasma processing method according to claim 1, wherein the member forreducing high frequency distribution is a dielectric.
 3. The plasmaprecessing method according to claim 1, wherein the processing is toform a deposited film on the object to be processed.
 4. The plasmaprocessing method according to claim 1, wherein the cathode electrode issubjected to geometric machining.
 5. The plasma processing methodaccording to claim 4, wherein the geometric machining is either ofaperture formation or slit formation.
 6. The plasma processing methodaccording to claim 1, wherein a soft magnetic material is used in aportion of the cathode electrode.
 7. The plasma processing methodaccording to claim 1, wherein the cathode electrode is a cylindricalelectrode.
 8. The plasma processing method according to claim 7, whereinthe object to be processed is cylindrical, and the object to beprocessed and the cathode electrode are coaxially arranged.
 9. Theplasma processing method according to claim 1, wherein the cathodeelectrode and the object to be processed are flat plates facing eachother.
 10. The plasma processing method according to claim 1, whereinthe cathode electrode has a length longer than the object to beprocessed.
 11. The plasma processing method according to claim 1,wherein the member for reducing high frequency distribution has a lengthlonger than the object to be processed.
 12. A plasma processing methodcomprising the steps of: housing an object to be processed in a vesselhaving a member for reducing high frequency distribution in a portionthereof and capable of being evacuated; reducing a pressure inside thevessel; supplying a gas into the vessel; supplying high frequency powerof 30 MHz to 300 MHz to a cathode electrode, the cathode electrode beingprovided outside the vessel, an empty space being provided between themember for reducing high frequency distribution and the cathodeelectrode, to generate a plasma between the member for reducing highfrequency distribution, the member having a thickness of 5 mm or more,and the object to be processed in the vessel, thereby performing plasmaprocessing for the object to be processed.
 13. The plasma processingmethod according to claim 12, wherein the member for reducing highfrequency distribution is a dielectric.
 14. The plasma processing methodaccording to claim 12, wherein the processing is to form a depositedfilm on the object to be processed.
 15. The plasma processing methodaccording to claim 12, wherein the cathode electrode is subjected togeometric machining.
 16. The plasma processing method according to claim15, wherein the geometric machining is either of aperture formation orslit formation.
 17. The plasma processing method according to claim 12,wherein a soft magnetic material is used in a portion of the cathodeelectrode.
 18. The plasma processing method according to claim 12,wherein the cathode electrode is a cylindrical electrode.
 19. The plasmaprocessing method according to claim 18, wherein the object to beprocessed is cylindrical, and the object to be processed and the cathodeelectrode are coaxially arranged.
 20. The plasma processing methodaccording to claim 12, wherein the cathode electrode and the object tobe processed are flat plates facing each other.
 21. The plasmaprocessing method according to claim 12, wherein the cathode electrodehas a length longer than the object to be processed.
 22. The plasmaprocessing method according to claim 12, wherein the member for reducinghigh frequency distribution has a length longer than the object to beprocessed.
 23. A plasma processing method comprising the steps of:housing an object to be processed in a vessel having a member forreducing high frequency distribution in a portion thereof and capable ofbeing evacuated; reducing a pressure inside the vessel; supplying a gasinto the vessel; supplying high frequency power of 30 MHz to 300 MHz toa cathode electrode, the cathode electrode being provided outside thevessel and being arranged so as to face the member for reducing highfrequency power distribution, the member having a thickness of 5 mm ormore, to supply an electromagnetic wave into the vessel through themember for reducing high frequency distribution, whereby a plasma isgenerated between the member for reducing high frequency distributionand the object to be processed in the vessel to perform plasmaprocessing for the object to be processed.
 24. The plasma processingmethod according to claim 23, wherein the member for reducing highfrequency distribution is a dielectric.
 25. The plasma processing methodaccording to claim 23, wherein the processing is to form a depositedfilm on the object to be processed.
 26. The plasma processing methodaccording to claim 23, wherein the cathode electrode is subjected togeometric machining.
 27. The plasma processing method according to claim26, wherein the geometric machining is either of aperture formation orslit formation.
 28. The plasma processing method according to claim 23,wherein a soft magnetic material is used in a portion of the cathodeelectrode.
 29. The plasma processing method according to claim 23,wherein the cathode electrode is a cylindrical electrode.
 30. The plasmaprocessing method according to claim 29, wherein the object to beprocessed is cylindrical, and the object to be processed and the cathodeelectrode are coaxially arranged.
 31. The plasma processing methodaccording to claim 23, wherein the cathode electrode and the object tobe processed are flat plates facing each other.
 32. The plasmaprocessing method according to claim 23, wherein the cathode electrodehas a length longer than the object to be processed.
 33. The plasmaprocessing method according to claim 23, wherein the member for reducinghigh frequency distribution has a length longer than the object to beprocessed.